package com.chinathinks.core.net.ntp;

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.InetAddress;

import com.chinathinks.core.net.DatagramSocketClient;

/***
 * The NTPUDPClient class is a UDP implementation of a client for the Network
 * Time Protocol (NTP) described in RFC 1305 as well as the Simple Network Time
 * Protocol (SNTP) in RFC-2030. To use the class, merely open a local datagram
 * socket with <a href="#open"> open </a> and call <a href="#getTime"> getTime
 * </a> to retrieve the time. Then call <a
 * href="org.apache.commons.net.DatagramSocketClient.html#close"> close </a> to
 * close the connection properly. Successive calls to <a href="#getTime">
 * getTime </a> are permitted without re-establishing a connection. That is
 * because UDP is a connectionless protocol and the Network Time Protocol is
 * stateless.
 * 
 * @author Jason Mathews, MITRE Corp
 * @version $Revision: 165675 $ $Date: 2005-05-02 15:09:55 -0500 (Mon, 02 May
 *          2005) $
 ***/

public final class NTPUDPClient extends DatagramSocketClient {
	/*** The default NTP port. It is set to 123 according to RFC 1305. ***/
	public static final int DEFAULT_PORT = 123;

	private int _version = NtpV3Packet.VERSION_3;

	/***
	 * Retrieves the time information from the specified server and port and
	 * returns it. The time is the number of miliiseconds since 00:00 (midnight)
	 * 1 January 1900 UTC, as specified by RFC 1305. This method reads the raw
	 * NTP packet and constructs a <i>TimeInfo</i> object that allows access to
	 * all the fields of the NTP message header.
	 * <p>
	 * 
	 * @param host
	 *            The address of the server.
	 * @param port
	 *            The port of the service.
	 * @return The time value retrieved from the server.
	 * @exception IOException
	 *                If an error occurs while retrieving the time.
	 ***/
	public TimeInfo getTime(InetAddress host, int port) throws IOException {
		// if not connected then open to next available UDP port
		if (!isOpen()) {
			open();
		}

		NtpV3Packet message = new NtpV3Impl();
		message.setMode(NtpV3Packet.MODE_CLIENT);
		message.setVersion(_version);
		DatagramPacket sendPacket = message.getDatagramPacket();
		sendPacket.setAddress(host);
		sendPacket.setPort(port);

		NtpV3Packet recMessage = new NtpV3Impl();
		DatagramPacket receivePacket = recMessage.getDatagramPacket();

		/*
		 * Must minimize the time between getting the current time, timestamping
		 * the packet, and sending it out which introduces an error in the delay
		 * time. No extraneous logging and initializations here !!!
		 */
		TimeStamp now = TimeStamp.getCurrentTime();

		// Note that if you do not set the transmit time field then originating
		// time
		// in server response is all 0's which is
		// "Thu Feb 07 01:28:16 EST 2036".
		message.setTransmitTime(now);

		_socket_.send(sendPacket);
		_socket_.receive(receivePacket);

		long returnTime = System.currentTimeMillis();
		// create TimeInfo message container but don't pre-compute the details
		// yet
		TimeInfo info = new TimeInfo(recMessage, returnTime, false);

		return info;
	}

	/***
	 * Retrieves the time information from the specified server on the default
	 * NTP port and returns it. The time is the number of miliiseconds since
	 * 00:00 (midnight) 1 January 1900 UTC, as specified by RFC 1305. This
	 * method reads the raw NTP packet and constructs a <i>TimeInfo</i> object
	 * that allows access to all the fields of the NTP message header.
	 * <p>
	 * 
	 * @param host
	 *            The address of the server.
	 * @return The time value retrieved from the server.
	 * @exception IOException
	 *                If an error occurs while retrieving the time.
	 ***/
	public TimeInfo getTime(InetAddress host) throws IOException {
		return getTime(host, NtpV3Packet.NTP_PORT);
	}

	/***
	 * Returns the NTP protocol version number that client sets on request
	 * packet that is sent to remote host (e.g. 3=NTP v3, 4=NTP v4, etc.)
	 * 
	 * @return the NTP protocol version number that client sets on request
	 *         packet.
	 * @see #setVersion(int)
	 ***/
	public int getVersion() {
		return _version;
	}

	/***
	 * Sets the NTP protocol version number that client sets on request packet
	 * communicate with remote host.
	 * 
	 * @param version
	 *            the NTP protocol version number
	 ***/
	public void setVersion(int version) {
		_version = version;
	}

}
